Utilization of biofuels

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Direct combustion

Short-rotation coppice (SRC) of certain fast-growing trees like willow can provide a bulk fuel for burning in power plants. It can be co-fired with coal to reduce emissions and prolong the life of coal-fired plants. Trees are planted at a rate of 15,000 per hectare and cut back to near ground level after a year. They usually re-grow as multiple stems and after two to four years can be harvested for fuel. The cycle is repeated over a life span of about 30 years.

One problem in a liberalized energy market is the compatibility between an agricultural process and an industrial process of producing electricity. Power producers have to guarantee power in 30-minute intervals at an agreed price and predicted four hours in advance. Failure to meet this challenge resulted in the demise of the ARBRE project in Yorkshire.

In the first edition of this book there was a section devoted to the ARBRE project (Arable Biomass Renewable Energy project), the UK's first biomass to biogas enterprise. It was designed to process short-rotation coppice willow and produce 10 MW of electricity. Early in 2003 the project was put into the hands of the receivers and 50 farmers, who had been encouraged to grow the necessary crops, were left without a market. This included a farm near Retford in Nottinghamshire owned by John Strawson, which had dedicated 11% of the farmland amounting to 172 ha to the crop. The two alternatives were either to abandon SRC altogether or find an alternative use for the crops. In opting for the second route most of the farmers agreed to revert to producing feedstock for thermal power stations and smaller market opportunities. A company called Renewable Energy Growers Ltd was formed in 2004 with 45 of the original growers. Renewable Energy Suppliers was formed shortly afterwards by John Strawson of Manor. He invented a novel mechanical process to reduce willow to the fine and consistent gauge of 3 to 10 mm that would enable it to be mixed with coal in a conventional power station. Granule sizes up 30 mm are also produced for a variety of installations as well as billets for domestic stoves and fires (see Fig. 8.1).

The energy density of the fuel is 15 gigajoules per tonne at a moisture content of 20-25%.

The willow is harvested in a three-year cycle and the 172 ha of willow yields 1720 oven-dried tonnes (ODT) per year to the coal-fired power station of Drax in Yorkshire and Cottam in Nottinghamshire under contract. Unfortunately, Drax recently declined to renew the contract. Altogether the Renewable Energy Supplies produced 3500 tonnes of willow in various forms including 'granules' in 2005-6. This is equivalent to a saving in carbon dioxide of 6125 tonnes. Renewable Energy Suppliers market their high-quality product as 'Koolfuel', which amounts to 75% of the total energy crop delivered to the energy markets in the UK.

Unlike most renewable energy sources, the economics of SRC willow compare favourably with conventional fuels. With recent price increases for fossil fuels it is likely that this is now the cheapest fuel. In the domestic sector even in 2003 it was the cheapest fuel.

Is this fuel truly carbon neutral? The standard equation is that the CO2 taken up in growth is returned to the atmosphere when burnt. However, some of the growth carbon is retained in the roots. On the other hand there is the carbon involved in the processing and transportation etc. The safest claim is that it is as near carbon neutral as any other form of renewable energy.

To maximize the opportunities presented by the SRC willow operation, Strawsons have developed a business park within the farm complex. After only one full year in operation the enterprise was now providing fuel for a number of enterprises with biomass heat boilers including business parks, schools, district and local heating installations and domestic developments.

Figure 8.1 Processing and cutting of willow, Manor Farm (courtesy of Renewable Energy Suppliers Ltd)

This is a qualified success story thanks to the persistence of the people involved in overcoming the many obstacles that are encountered whenever novel enterprises seek to break the mould.

The UK government is currently supporting a project to grow elephant grass - a giant tropical plant reaching up to 3.5 m - ultimately over 180,000 ha. A power station to burn this type of crop is nearing completion in Eccleshall, Staffordshire. Drax is also earmarked to use this crop and farmland around Drax will supply it. The question arises as to whether this constitutes a direct threat to SRC willow since there is possibly a limit to which a large coal-fired plant like Drax can accommodate biofuels.

Energy from wood waste

At the moment the majority of waste from forest maintenance, furniture manufacture and other timber-related activities is not exploited for energy. At Renewable Energy Suppliers it is utilized, creating wood pellets in a compact and easy-to-use form of biomass fuel suitable for domestic and district heating. This is a near carbon neutral source of energy produced by compressing wood waste under high pressure into cylinders or billets about 10 cm in diameter and 10-30 cm long. They have a low moisture content, giving them a high energy density.

Such manufacturing plants range from 100 tonnes to 10,000 tonnes a year capacity. Billets are produced from a variety of feedstocks but the most effective are shavings and sawdust. However, other feedstocks like tree bark straw and other crop residues can be used, though they are more suitable for industrial scale plants than domestic use. There is a growing market in domestic pellet stoves and boilers, especially in Germany and Austria. An Austrian 12 kW central heating pellet boiler is designed to operate in conjunction with solar thermal heating. In Scandinavia, pellets are widely used for district heating. In Denmark there is a power station that uses 300,000 wood pellets a year to produce 570 MWe (electricity) and 570MWth (heat) a year. Such plants commonly derive 50% of their energy from pellets to achieve up to 95% efficiency. As the price of oil crosses the $70 a barrel threshold so wood pellets become increasingly cost-effective - even in the UK.

Work began early in 2006 on a £90 million biomass heat and power plant by the power company E.oN at Steven's Croft near Lockerbie, Scotland. It will burn 220,000 tonnes per year of forest waste, producing enough power for about 70,000 homes. In this connection the Deputy First Minister for Scotland affirmed that his country 'has abundant resources to lead the way in biomass development in the UK, providing and sustaining jobs and meeting local energy needs . . . . we can help make Scotland a powerhouse for renewable energy'.

A perennial problem facing renewable energy in all its forms is the planning regime. In one instance a farmer in Cornwall converted to producing rapid rotation crops for a thermal plant that was due to be built nearby. For crops to be available requires several years of lead-in time. In the event, the plant was refused planning permission, which highlights a problem that bedevils the whole renewables spectrum.

On the positive side a 36 MW straw burning plant, the Elan Power Station in Cambridgeshire, is the largest in the world and 'a noted success'.1 Equally successful are three thermal energy plants in the UK, burning mostly poultry litter with a total capacity of 64.7 MW

Poultry litter, which is a mixture of chicken manure and straw or wood shavings, is a viable form of fuel for electricity generation in rural areas. The largest plant in the UK is a 39 MW installation in Scotland.

Bioenergy is a reliable source of power and, as such, can act as back-up or 'spinning reserve' for intermittent renewables. 'Improved financial and political support from governments is, however, of great importance to the commercial success of the whole biomass industry both in terms of generating investor confidence and ensuring that plants are economically viable on start-up'.2

Municipal solid waste

The increasing cost of landfill disposal of household waste and the growing scarcity of sites is encouraging the incineration of municipal solid waste (MSW) which is increasingly being used to generate both heat and power on a municipal scale. Examples include Sheffield Heat and Power, which generates electricity for the grid whilst providing low-grade heat for the city centre, the universities and hospitals. Another example is the South East London Combined Heat and Power (CHP) Plant designed to burn 420,000 tonnes of MSW a year, producing steam for a 31 MW turbo-generator.

There have been concerns about emissions from incinerators such as heavy metals and organic compounds such as dioxins. However, the EU is imposing increasingly stringent emissions standards. Currently energy from waste accounts for 0.1% of all UK dioxin emissions.

The burning of biomass to generate steam to drive turbines for combined heat and power is the most common method of deriving energy either from waste or energy crops. However, gasification is the preferred method for large-scale systems of >10 MW Conversion of coal-fired plants which can co-fire biomass is a medium-term economic option.

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